On-chip high capacitance termination for transmitters

ABSTRACT

A modulator and a capacitor are integrated on a semiconductor substrate for modulating a laser beam. Integrating the capacitor on the substrate reduces parasitic inductance for high-speed optical communication.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional PatentApplication No. 15/656,137, filed on Jul. 21, 2017, entitled “On-ChipHigh Capacitance Termination For Transmitters,” which claims priority toU.S. Provisional Patent Application No. 62/365,862, filed on Jul. 22,2016, entitled “On-Chip High Capacitance Termination For Transmitters,”the disclosures of which are hereby incorporated by reference in theirentirety for all purposes.

BACKGROUND

This application relates to integrating optical and electricalcomponents on a silicon substrate. Silicon integrated circuits (“ICs”)have dominated the development of electronics and many technologiesbased upon silicon processing having been developed over the years.Their continued refinement led to nano-scale feature sizes that can beimportant for making CMOS (complementary metal-oxide-semiconductor)circuits. On the other hand, silicon is not a direct-bandgap material.Although direct-bandgap materials, including III-V compoundsemiconductor materials, have been developed, there is a need in the artfor improved methods and systems related to photonic ICs utilizingsilicon substrates.

BRIEF SUMMARY

Embodiments of the present invention provide devices, systems, and/ormethods of a composite device, such combining functionality of twodifferent semiconductor materials to create an optical device. Acapacitor is mounted on a silicon-photonics device to reduce impedancemismatch between a driver and a modulator. The capacitor is mounted onthe silicon-photonics device to reduce parasitic inductance during highfrequency modulation. In some embodiments, a method to improve RFperformance of an EAM device by integrating a high capacitance componentand solid ground planes very close to the EAM device is disclosed (e.g.,within 20, 50, 100, 125, 200, 500, and/or 1,000 μm and/or not closerthan 0.1, 1, 2, and/or 5 μm).

In some embodiments, a device for modulating light from a semiconductorlaser comprises: a platform, wherein: the platform comprises asubstrate, the substrate is made of a first semiconductor material, theplatform comprises a first optical waveguide integrated on thesubstrate, and/or the platform comprises a ground plane integrated onthe substrate; a chip, wherein: the chip is integrated on the substrate,the chip is made of a second semiconductor material, the chip comprisesa second optical waveguide, and/or the second optical waveguide isoptically aligned with the first optical waveguide; a contact, wherein:the contact is integrated on the substrate, and/or the contact iselectrically connected with the chip; and/or a capacitor, wherein: thecapacitor is integrated on the substrate, and/or the capacitor iselectrically connected with the contact. In some embodiments, the devicefurther comprises a resistor integrated on the substrate electricallybetween the contact and the capacitor. In some embodiments, the chip iselectrically between the contact and the ground plane; the capacitor iselectrically between the contact and the ground plane; the contact iselectrically connected with the chip via a vertical plug; the chip isintegrated on the substrate by being bonded to the platform; the chip isintegrated on the substrate by being bonded to the substrate; the chipis bonded in a recess of the platform; the second optical waveguide isoptically aligned with the first optical waveguide by butt coupling; thechip comprises multi-quantum wells; the capacitor has a capacitancebetween 50 and 10,000 picofarads; the substrate is mounted on a printedcircuit board; and/or the capacitor is within 20, 50, 100, 200, 500,and/or 1,000 μm of the chip.

In some embodiments, a method for creating a modulator for asemiconductor laser comprises: bonding a chip to a platform, wherein theplatform comprises a substrate; forming a contact integrated on thesubstrate; forming a ground plane integrated on the substrate, whereinthe chip is electrically between the contact and the ground plane;and/or bonding a capacitor to the platform, wherein: the capacitor isintegrated on the substrate, and/or the capacitor is electricallybetween the contact and the ground plane. In some embodiments, themethod comprises applying a modulation signal to the contact, whereinthe chip is used as an electro absorption modulator. In someembodiments, the capacitor has a capacitance between 500 and 900picofarads.

In some embodiments, a method for modulating a semiconductor modulatorcomprises: applying a modulation signal to a contact, wherein thecontact is integrated on a substrate; transmitting a first part of themodulation signal to a semiconductor structure, wherein: thesemiconductor structure has a direct bandgap, the semiconductorstructure is integrated on the substrate, the semiconductor structurecomprises an optical waveguide, and/or the first part of the modulationsignal applied to the semiconductor structure modulates an optical beamin the optical waveguide of the semiconductor structure; and/or shuntinga second part of the modulation signal through a capacitor that isintegrated on the substrate. In some embodiments, the method comprises:bonding the semiconductor structure to the substrate to integrate thesemiconductor structure on the substrate; forming the contact integratedon the substrate; forming a ground plane integrated on the substrate,wherein the semiconductor structure is electrically between the contactand the ground plane; and/or bonding the capacitor to the substrate,wherein the capacitor is electrically between the contact and the groundplane. In some embodiments, the modulation signal has a frequencybetween 2 and 100 gigahertz; the modulation signal modulates an opticalbeam guided in the semiconductor structure; and/or the capacitor iswithin 200 μm of the semiconductor structure.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating various embodiments, are intended for purposes ofillustration only and are not intended to necessarily limit the scope ofthe disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described in conjunction with the appendedfigures.

FIG. 1 depicts an embodiment of a simplified cross-sectional side viewof a chip bonded to a substrate.

FIG. 2 depicts an embodiment of a simplified cross-sectional side view acapacitor integrated on the substrate.

FIG. 3 depicts an embodiment of a simplified top view of the capacitorintegrated on the substrate.

FIG. 4 illustrates a flowchart of an embodiment of a process forcreating a modulator for a semiconductor laser.

FIG. 5 illustrates a flowchart of an embodiment of a process formodulating a semiconductor modulator.

FIG. 6 depicts a simplified diagram of an embodiment of a substrateintegrated on a PCB.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DESCRIPTION

The ensuing description provides preferred exemplary embodiment(s) only,and is not intended to limit the scope, applicability, or configurationof the disclosure. Rather, the ensuing description of the preferredexemplary embodiment(s) will provide those skilled in the art with anenabling description for implementing a preferred exemplary embodiment.It is understood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

Embodiments relate generally to a platform bonded to a chip to form acomposite device. For example, a platform (e.g., a silicon platform) canbe bonded to a semiconductor of different material (e.g., III-V). Thoughmaking devices with silicon has some advantages (e.g., cost anddeveloped fabrication methods), silicon is not a direct-bandgapmaterial. In certain applications, it is desirable to have adirect-bandgap material (e.g., for a laser gain medium). Thus, a chipmade of a semiconductor material having a direct bandgap is integratedwith a silicon platform.

Achieving high Radio Frequency (RF) performance at high frequency, suchas 1 GHz to 50 GHz, 500 GHz, or beyond, on silicon photonics chipscontaining an EAM (electro absorption modulator) device is challenging,especially with high-level of integration of EAM devices. The challengecomes from an impedance mismatch between an EAM device (e.g., built fromIII-V material) and a 50 ohms impedance line carrying RF signals from adriver to the EAM devices

Some efforts can be done to improve impedance matching and termination.For example, adjusting a layout of the PCB (printed circuit board)and/or adding one or more resistors and/or capacitors on the PCB boardcan help improve impedance matching. However, in those efforts, EAMchips are very close to the PCB board, and a high-density of modulatordevices is difficult to achieve due to space constraints. Some III-Vdesigns and/or silicon-photonics designs lack optimization of RFgrounding and/or modulator termination impedance.

In some embodiments, one or more of the following are achieved: improvedand/or optimized RF layout for better high frequency EAM eyes; improvedgrounding surrounding EAM devices; and/or improved termination impedancewith high capacitance close to an EAM device.

Referring first to FIG. 1, a platform 100 is shown. The platform 100comprises a base layer 104, a lower layer 108 on top of the base layer104, a device layer 112 on top of the lower layer 108, and an upperlayer 116 on top of the device layer 112. The base layer 104 is made ofa first semiconductor material. For example, in some embodiments, thebase layer 104 is a crystalline silicon substrate; the lower layer 108is an oxide layer (e.g., SiO2); the device layer 112 is made ofcrystalline silicon; and the upper layer 116 is an oxide layer (e.g.,SiO2). In some embodiments, the base layer 104, the lower layer 108, andthe device layer 112 started as a silicon on insulator (SOI) wafer(e.g., the base layer 104 is a handle; the lower layer 108 is a buriedoxide layer (BOX); and the device layer 112 is a device layer of the SOIwafer). In this embodiment, the device layer 112 has been processed(e.g., waveguide(s), mirror(s), and/or grating(s) have been formed inthe device layer 112) and the upper layer 116 covers the device layer112 (e.g., as an upper cladding) to create the platform 100. In someembodiments, the device layer 112 comprises a first waveguide 118, andthe lower layer 108 and the upper layer 116 act as cladding material forthe waveguide (the device layer 112 having a higher index of refractionthan the lower layer 108 and the upper layer 116).

A chip 120 is bonded to the platform 100. In some embodiments, the chip120 is bonded to the base layer 104. In some embodiments, the base layer104 is referred to as a substrate. In some embodiments, the chip 120 isbonded in a recess of the platform. The chip 120 comprises an activeregion 124. In some embodiments, the chip 120 comprises a secondwaveguide 126 (e.g., in the active region 124). An arrow shows adirection of beam propagation 128 from left to right.

In some embodiments, the chip 120 is bonded to the platform 100 asdescribed in U.S. patent application Ser. No. 14/509,914, filed on Oct.8, 2014, and/or U.S. patent application Ser. No. 15/592,704, filed onMay 11, 2017, the disclosure of each is incorporated by reference forall purposes.

The first waveguide 118 is butt-coupled with the second waveguide 126.In some embodiments, a third waveguide 130 couples the first waveguide118 with the second waveguide 126. In some embodiments, the thirdwaveguide 130 is referred to as an optical bridge. In some embodimentsthe third waveguide is formed as described in U.S. patent applicationSer. No. 14/996,001, filed on Jan. 14, 2016 and/or U.S. patentapplication Ser. No. 15/426,366, filed on Feb. 7, 2017, the disclosureof each is incorporated by reference for all purposes.

FIG. 2 depicts an embodiment of a simplified cross-sectional side view acapacitor 200 integrated on the substrate (e.g., integrated on the baselayer 104). The cross section is rotated by 90 degrees compared to FIG.1 so that the direction of beam propagation 128 is coming out of thepage. A contact 204 is integrated on the substrate. The contact 204 iselectrically connected with the chip 120. In some embodiments, a firstplug 208-1 is formed through a via connecting the contact 204 with thechip 120.

The capacitor 200 is electrically connected with the contact 204. Insome embodiments, a resistor 212 electrically connects the contact 204with the capacitor 200. The capacitor 200 is bonded to a first pad216-1. In some embodiments, a conductive epoxy 220 is applied to attachthe capacitor 200 to the first pad 216-1. A second plug 208-2 connectsthe contact 204 with the resistor 212. A third plug 208-3 connects theresistor 212 with the first pad 216-1.

A first ground plane 224-1 and/or a second ground plane 224-2 areintegrated on the substrate. The first ground plane 224-1 iselectrically connected with the second ground plane 224-2 with a fourthplug 208-4, a fifth plug 208-5, and/or more plugs. In some embodiments,the first ground plane 224-1 and the second ground plane 224-2 haveoverlapping areas to further increase capacitance. In some embodiments,the first ground plane 224-1 is not used. The chip 120 is electricallyconnected with the second ground plane 224-2 via a sixth plug 208-6.

The capacitor 200 is electrically connected with the second ground plane224-2 at a second pad 216-2 by a wire bond 228. The first ground plane224-1 is formed as a first metal layer. The second ground plane 224-2,the first pad 216-1, and/or the contact 204 are formed concurrently as asecond metal layer. Insulating material 232 (e.g., SiO2) is used toelectrically insulate features and/or to provide material forintegrating features (e.g., the first metal layer, the second metallayer, plugs 208, and/or the resistor 212) onto the substrate (e.g.,base layer 104).

In some embodiments, the chip 120 and the capacitor 200 integrated onthe substrate form an optical device 236 for modulating light foroptical communication. In some embodiments, the chip 120 comprises adirect bandgap material (e.g., III-V or II-VI material, such as InP orGaAs). The active region 124 comprises a multi-quantum well structure.

In some embodiments, a high capacitance capacitor is used (e.g.,50-10,000, 100-5,000, 200-1000, 400-900, 500-750, 600, 650, 660, 670,680, 690, and/or 700 picofarad capacitor). In some embodiments, thecapacitor 200 fits on an area equal to or less than 225 mm². A highcapacitance capacitor is not easily integrated on an electronics chip.Plugs 208 are used to create robust connections. The capacitor 200provides high on-chip capacitance for EAM termination.

FIG. 3 depicts an embodiment of a simplified top view of the capacitor200 integrated on the substrate. In some embodiments, the substrate isintegrated on a PCB. The optical device 236 comprises a first RF bondpad 304-1, a second RF bond pad 304-2, and third RF bond pad 304-3. Thefirst RF bond pad 304-1 connects to the contact 204. The second RF bondpad 304-2 and/or the third RF bond pad 304-3 connect to the secondground plane 224-2. A signal is applied to the first bond pad 304-1. Thesignal changes electrical potential and/or current through the chip 120,which is used to modulate an optical beam traveling through the chip120. In some embodiments, a metal ridge extends on top of the secondwaveguide 126 in the direction of beam propagation. In some embodiments,vertical is a direction looking into or out of the page in FIG. 3 (e.g.,a direction normal to an un-etched surface of the substrate. In someembodiments, plugs 208 are vertical plugs (e.g., plug 208 extendsvertically to connect a first component separated by a vertical distancefrom a second component). Horizontal is orthogonal to vertical andorthogonal to the direction of beam propagation 128.

FIG. 4 illustrates a flowchart of an embodiment of a process 400 forcreating a modulator for a semiconductor laser. Process 400 begins instep 404 with bonding the chip 120 to the platform 100. The platform 100comprises a substrate (e.g., base layer 104). In step 408, a contact(e.g., contact 204) is formed, wherein the contact 204 is integrated onthe substrate. In step 412, a ground plane is formed, wherein the groundplane is integrated on the substrate. The chip 120 is electricallybetween the contact 204 and the ground plane 224. In some embodiments,steps 408 and 412 are performed concurrently. In step 416, a capacitoris bonded to the platform 100, integrating the capacitor on thesubstrate. The capacitor is electrically between the contact 204 and theground plane 224.

FIG. 5 illustrates a flowchart of an embodiment of a process 500 formodulating a semiconductor modulator. A semiconductor laser generates anoptical beam. In some embodiments, the optical beam is generated usinganother chip integrated on the base layer 104.

The optical beam is guided to the chip 120 by the first waveguide 118.In step 504, a modulation signal is applied to the contact 204. Thecontact 204 is integrated on the substrate. A first part of themodulation signal is transmitted to a semiconductor structure (e.g.,chip 120), step 508. The semiconductor structure has a direct bandgap.The semiconductor structure is integrated on the substrate. Thesemiconductor structure comprises an optical waveguide (e.g., the secondwaveguide 126). The first part of the modulation signal modulates anoptical beam in the optical waveguide. In step 512, a second part of themodulation signal is shunted to a capacitor (e.g., capacitor 200) thatis integrated on the substrate.

In some embodiments, the capacitor 200 is used to mitigate impedancemismatch. By having the capacitor 200 integrated on the substrate, andthe wire bond 228 grounded to the ground plane 224 (wherein the groundplane 224 is integrated on the substrate), a length of the wire bond 228is shortened (e.g., as compared to having the capacitor 200 off thesubstrate). A wire bond 228 that is shorter helps mitigate parasiticinductance in the wire bond 228. Parasitic inductance increases withincreased frequency. In some embodiments, an RF frequency between 1 and500 GHz, between 5 and 150 GHz, between 10 and 60 GHz, and/or between 20and 30 GHz is applied to the contact 204 (e.g., 25 GHz, 50 GHz).

In some embodiments, a device (e.g., for modulating a semiconductormodulator) comprises a substrate; a semiconductor structure, wherein:the semiconductor structure has a direct bandgap, and the semiconductorstructure is integrated on the substrate; and a capacitor integrated onthe substrate. For example, instead of bonding the chip 120 to thesubstrate, a semiconductor structure (e.g., multi-quantum wells) isgrown on the substrate (e.g., grown on an InP or GaAs substrate).

FIG. 6 depicts a simplified diagram of an embodiment of a substrate 604integrated on a PCB 608. In some embodiments, the substrate 604 is thebase layer 104. An EAM device 612 (e.g., comprising the chip 120) isintegrated on the substrate 604. The capacitor 200 is integrated on thesubstrate 604. A laser 616 is integrated on the substrate 604. Lightfrom the laser 616 is transmitted through the first waveguide 118 (whichis also integrated on the substrate 604) to the second waveguide 126 ofthe chip 120, which is part of the EAM device 612. A driver 620 is usedto generate an electrical signal to send to the EAM device 612. Lightfrom the laser 616 is modulated at the EAM device 612 and transmitted toa third waveguide 624. In some embodiments, the third waveguide 624 is asemiconductor waveguide (e.g., made of crystalline silicon in the devicelayer 112). An optical coupler 628 expands an optical beam from thethird waveguide 624 to couple with an optical fiber 632. An example ofan optical coupler 628 from a semiconductor waveguide to an opticalfiber is given in U.S. patent application Ser. No. 14/615,942, filed onFeb. 6, 2015, which is incorporated by reference for all purposes.

The above description of exemplary embodiments has been presented forthe purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form described, andmany modifications and variations are possible in light of the teachingabove. For example, in some embodiments the chip 120 is used as amodulator as described in U.S. patent application Ser. No. 15/426,823,filed on Feb. 7, 2017, the disclosure of which is incorporated byreference. Thus, the optical device 236 could comprise multiplemodulators (e.g., two, four, and or eight) with multiple capacitors 200(e.g., two, four, and/or eight capacitors). In some embodiments, thecapacitor 200 is integrated on the substrate in lieu of, or in additionto, sharp bends in waveguides as described in the '823 application.

The embodiments were chosen and described in order to explain theprinciples of the invention and its practical applications to therebyenable others skilled in the art to utilize the invention in variousembodiments and with various modifications as are suited to theparticular use contemplated.

The specific details of particular embodiments may be combined in anysuitable manner without departing from the spirit and scope ofembodiments of the invention. However, other embodiments of theinvention may be directed to specific embodiments relating to eachindividual aspect, or specific combinations of these individual aspects.

Also, it is noted that the embodiments may be described as a processwhich is depicted as a flowchart, a flow diagram, a data flow diagram, astructure diagram, or a block diagram.

Although a flowchart may describe the operations as a sequentialprocess, many of the operations can be performed in parallel orconcurrently. In addition, the order of the operations may bere-arranged. A process is terminated when its operations are completed,but could have additional steps not included in the figure. A processmay correspond to a method, a function, a procedure, a subroutine, asubprogram, etc.

A recitation of “a”, “an”, or “the” is intended to mean “one or more”unless specifically indicated to the contrary.

All patents, patent applications, publications, and descriptionsmentioned here are incorporated by reference in their entirety for allpurposes. None is admitted to be prior art.

1. A method for modulating a semiconductor modulator, the methodcomprising: applying a modulation signal to a contact, wherein thecontact is integrated on a substrate; transmitting a first part of themodulation signal to a semiconductor structure, wherein: thesemiconductor structure has a direct bandgap, the semiconductorstructure is integrated on the substrate, the semiconductor structurecomprises an optical waveguide, the optical waveguide is a semiconductorwaveguide, and the first part of the modulation signal applied to thesemiconductor structure modulates an optical beam in the opticalwaveguide of the semiconductor structure; and shunting a second part ofthe modulation signal through a capacitor that is integrated on thesubstrate.
 2. The method of claim 1, further comprising: bonding thesemiconductor structure to a platform, wherein the platform comprisesthe substrate, to integrate the semiconductor structure on thesubstrate; forming the contact integrated on the substrate; forming aground plane integrated on the substrate, wherein the semiconductorstructure is electrically between the contact and the ground plane; andbonding the capacitor to the platform, wherein the capacitor iselectrically between the contact and the ground plane.
 3. The method ofclaim 2, wherein: the optical waveguide is a first optical waveguide;the platform comprises a second optical waveguide; the first opticalwaveguide is disposed in a first semiconductor material; the secondoptical waveguide is disposed in a second semiconductor material,different from the first semiconductor material; and the method furthercomprises transmitting the optical beam from the first optical waveguideinto the second optical waveguide.
 4. The method of claim 1, wherein themodulation signal has a frequency between 2 and 100 gigahertz.
 5. Themethod of claim 1, wherein the capacitor is within 200 μm of thesemiconductor structure.
 6. The method of claim 3, wherein a thirdoptical waveguide is disposed in the second semiconductor material ofthe platform, and the method further comprises transmitting the opticalbeam from the third optical waveguide into the first optical waveguide.7. The method of claim 6, wherein the second semiconductor material iscrystalline silicon.
 8. The method of claim 7, wherein the firstsemiconductor material is a III-V compound.
 9. The method of claim 6,further comprising expanding the optical beam using an optical couplerintegrated on the substrate.
 10. The method of claim 1, furthercomprising generating the optical beam using a laser integrated on thesubstrate.
 11. A method for modulating a semiconductor modulator, themethod comprising: applying a modulation signal to a contact, wherein:the contact is integrated on a substrate; the substrate is part of aplatform; the platform is made of a first semiconductor material; theplatform comprises a ground plane; the platform comprises a firstoptical waveguide disposed in the first semiconductor material andintegrated on the substrate; and the first optical waveguide is asemiconductor optical waveguide; transmitting a first part of themodulation signal to a semiconductor structure, wherein: thesemiconductor structure is integrated on the substrate; thesemiconductor structure has a second semiconductor material; the secondsemiconductor material has a direct bandgap; the semiconductor structureis integrated on the substrate; the semiconductor structure comprises asecond optical waveguide; the second optical waveguide is disposed inthe second semiconductor material; the second optical waveguide is asemiconductor waveguide; the second optical waveguide is opticallyaligned with the first optical waveguide; the contact is electricallyconnected with the semiconductor structure; the first part of themodulation signal applied to the semiconductor structure modulates anoptical beam in the second optical waveguide of the semiconductorstructure; and shunting a second part of the modulation signal through acapacitor that is integrated on the substrate, wherein the capacitor iselectrically connected with the contact.
 12. The method of claim 11,wherein the modulation signal has a frequency between 2 and 100gigahertz.
 13. The method of claim 11, wherein the capacitor is within200 μm of the semiconductor structure.
 14. The method of claim 11,wherein the semiconductor structure is electrically integrated on thesubstrate between the contact and the ground plane.
 15. The method ofclaim 11, wherein the capacitor has a capacitance between 500 and 900picofarads.
 16. A method for modulating a laser beam, the methodcomprising: generating an optical beam using a laser, wherein: the laseris integrated on a substrate, and the substrate is mounted on a printedcircuit board; guiding the optical beam, using a first optical waveguideintegrated on the substrate, from the laser to a second opticalwaveguide in a chip, wherein: the first optical waveguide is in aplatform; the second optical waveguide is optically aligned with thefirst optical waveguide by butt coupling; and the chip is integrated onthe substrate; applying a modulation signal to a contact, wherein thecontact is integrated on the substrate; transmitting a first part of themodulation signal to the chip, wherein: the chip has a direct bandgap;the first part of the modulation signal applied to the chip modulatesthe optical beam in the second optical waveguide to generate a modulatedbeam; shunting a second part of the modulation signal through acapacitor that is integrated on the substrate; and coupling themodulated beam to a third optical waveguide, wherein the third opticalwaveguide is in the platform.
 17. The method of claim 16, wherein: thechip is electrically between the contact and a ground plane, the groundplane is integrated on the substrate, and the capacitor is electricallybetween the contact and the ground plane.
 18. The method of claim 16,further comprising a resistor electrically integrated on the substratebetween the contact and the capacitor.
 19. The method of claim 16,wherein the chip is bonded in a recess of the platform so that the chipis closer to the substrate than the first optical waveguide is to thesubstrate.
 20. The method of claim 16, wherein the capacitor has acapacitance between 500 and 900 picofarads.